There is no full-feature .Blend to .MDX converter out there, and most that exist are very clunky and barely functional. You'll either have to get Gmax and download NeoDex, though I don't recommend it unless you already have a lot of experience in Gmax or 3Ds Max as they're very difficult to learn programs, or buy a copy of Milkshape, which honestly I've never used Milkshape but it's actually a really cheap program that's only like 50 bucks as apposed to the 1000s for 3Ds Max. Milkshape natively supports both WC3 and Source Engine MDX format fully, or at least enough to get a good model that you can export and finish in Maggos or MDLVis. (Mentioning that because it could be a bit confusing, but just rest assured it does support WC3)